Add step to setup for building cross arch containers

[yeazelm]

The nvidia-smoke tests need multi-arch builds if using the README which is missing a command to fully configure the remote builder. This should enable the builds to succeed.

Testing done:

The buildx command failed to find arm platforms until this command was run, now it shows all the required platforms:

$ docker buildx ls
NAME/NODE           DRIVER/ENDPOINT                                               STATUS  BUILDKIT             PLATFORMS
gracious_shirley *  docker-container
  gracious_shirley0 unix:///var/run/docker.sock                                   running v0.12.3              linux/amd64, linux/amd64/v2, linux/amd64/v3, linux/amd64/v4, linux/386
  gracious_shirley1 ssh://fedora@HOSTNAME running v0.12.3              linux/arm64, linux/arm/v7, linux/arm/v6
default             docker
  default           default                                                       running v0.11.6+616c3f613b54 linux/amd64, linux/amd64/v2, linux/amd64/v3, linux/amd64/v4, linux/386

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.